1. Field of the Invention
The present invention generally relates to a speech codec. More specifically, the present invention relates to a speech codec of a CELP system in which a speech signal is compressed and coded.
2. Description of the Prior Art
In recent years, researches of speech coding technique for coding a speech signal with compression are actively made, and a speech codec at low bit rate is being rapidly put into practical use in a field of communication such as a mobile communication system, and a field of speech storage.
As a speech coding system at a low bit rate being put into practice presently, a CELP system ("CODE-EXCITED LINEAR PREDICTION (CELP) HIGH-QUALITY SPEECH AT VERY LOW BIT RATES": Proc. ICASSP pp 937-940 (1985)) of a degree of 8 kbps is known, and an improvement of a VSELP system (VECTOR SUM EXCITED LINEAR PREDICTION) developed by Motorola Inc. is being tried.
A speech codec adopting the CELP system is performed in accordance with the following steps basically:
(1) a drive sound source generating step for generating a predetermined drive sound source signal,
(2) a speech synthesizing step for synthesizing a speech signal on the basis of the drive sound source signal generated in the drive sound source generating step, and
(3) a code outputting step for comparing a synthesized speech signal that is synthesized in the speech synthesizing step and an inputted speech signal with each other and for selectively outputting a code corresponding to a drive sound source at a timing that an error between the both is minimum.
However, it is a fact that in a speech coding system at low bit rate less than 4 kbps, a sufficient speech quality is not obtained in such the CELP system or the VSELP system. A reason is considered that a semi-periodic pitch pulse of a voiced sound in the above described step (3) can not be sufficiently reproduced, and therefore, the quality is deteriorated.